Conventional polyolefin-based adhesive resins for bonding to or bonding together polyolefins and polar materials such as nylon, ethylene vinyl alcohol copolymer, metals and the like, are made using multiple step processes. First, an olefin, such as ethylene, commonly in gaseous form, is polymerized or co-polymerized with other monomers to form a polyolefin and extruded into pellets as a finished form.
Second, at least some polyolefin thus prepared must be further chemically reacted with a chemical having a polar functional group to provide a modified (“grafted”) polyolefin having a polar functionality (herein referred to as a “graft”). One way of performing this step is to visbreak the polyolefin in the molten state under conditions of high shear and/or temperature, in the presence of the polar monomer, to cause formation of free radicals that then react with the polar monomer. Another way is to dissolve the polyolefin in a solvent along with the polar monomer in the presence of a peroxide catalyst or other suitable catalyst that facilitates chemical grafting of the monomer onto the polyolefin in solution. Either process results in a polyolefin grafted with a polar monomer. The graft copolymer thus prepared is then typically pelletized in an extruder.
Third, the graft copolymer is typically melt-blended with an additional quantity of polyolefin to dilute the graft copolymer to a desired concentration, and to provide a polyolefin-based adhesive resin that has processing and physical properties suitable for the end use application. The mixing is usually performed by melting the polyolefin pellets and the graft pellets above the melting point of the two components and mixing the melted materials to desirably obtain a homogenous product. This additional melt blending is yet another expense. The polyolefin-based adhesive resin thus prepared is then pelletized from an extruder.
There is a need for a less expensive, less complicated process for producing polyolefin-based adhesive resins. There is also a need for a better quality polyolefin-based adhesive resin.
An example of a process for producing polyolefin-based adhesive resin is described in U.S. Pat. No. 4,487,885, issued Dec. 11, 1984. The process described therein utilizes a major amount of polyolefin polymer or polymers, which, as described above, has been formed by polymerizing an olefin or olefins and extruded into pellets as a finished form. The pelletized polymer or polymers are next mixed with graft and heated to above the melting point of the components under high shear. A heated extruder may be used to accomplish the latter step, and the melt mixed product can be recovered in the form of pellets. As noted in the patent, the product of the process may consist of from about 70–99.5 wt. % of polyolefin, e.g. polyethylene, and about 0.05–30 wt. % of the graft.
While conventional processes for producing polyolefin-based adhesive resins have been found to be useful, there are several disadvantages inherent in those processes. For example, in heating and shearing the polymerized polyolefin, e.g. polyethylene, usually in the form of pellets, above its melting point, imperfections, usually in the form of gelled polymer, are formed with each such heat history. The least amount of such imperfections is desired so that the adhesive resin when applied to a substrate will be continuous and without visible and/or functional imperfections.
Additionally, the conventional processes described above are costly due to the additional equipment and the energy required to first polymerize the olefin monomer, pelletize the polyolefin, and then melt and mix the formed polyolefin and graft material to form the adhesive product.
Thus there is a need for an improved process for producing polyolefin-based adhesive resins which reduces the amount of imperfections, such as gelled polymer, of the polyolefin material by eliminating one melt processing and extrusion step after the polymerization. There is also a need for a process that reduces the time, energy and equipment required to produce the desired polyolefin-based adhesive resins.